1. Field of the Invention
This invention concerns improvements in and relating to providing information, particularly, but not exclusively, on the dispersion of radioactive material within a matrix.
2. Present State of the Art and Summary of the Invention
The establishment of the amount of radioactive material within a matrix is useful in a number of situations. Particularly in the context of uranium, the ability of the radioactive material to shield against its own emissions is a potential source of error. At present this is addressed by considering the measured result in a pessimistic manner. However, undue pessimism causes increased cost and exposure to dose considerations.
The present invention has amongst its potential aims to indicate whether self-shielding is an issue for radioactive material within a matrix.
According to a first aspect of the present invention we provide a method for providing information on the dispersion of a radioactive material within a matrix, the method comprising:                measuring emissions of a first energy, the emissions including emissions of a first type, to provide a first type value;        measuring emissions of a second energy, the emissions including emissions of a second type, to provide a second type value;        measuring emissions of a third energy, the emissions including emissions of a third type, to provide a third type value;        the interrelationship of the values for the first type value, second type value and third type value providing information on the dispersion of the radioactive material within the matrix.        
The information on the dispersion may be an indication that the radioactive material is dispersed or an indication that the radioactive material is not dispersed. The information on the dispersion may be an indication that the radioactive material can be considered to be dispersed or not be considered to be dispersed. The information may provide a measure of the dispersion. The information may be used to confirm and/or question information on dispersion from another methodology, for instance differential peak analysis.
The radioactive material may be uranium and/or an isotope thereof. The uranium may be present along with one or more other elements or isotopes of other elements. The method may provide information on the dispersion of one or more other elements or isotopes of other elements, but need not.
The matrix may be waste. The matrix may be of lower density and/or lower atomic number than the radioactive material. The matrix may be present in a container, for instance a drum or crate.
The emissions may be measured by one or more detectors.
Preferably the first energy is 98.44 keV. The emissions of the first energy may be measured by measuring a range of energies. Preferably the range of energies includes 98.44 keV. The range of energies may have a lower limit of 96.9 keV±0.5 keV. The range of energies may have a lower limit of 96.9 keV±0.25 keV. The range of energies may have a lower limit of 96.9 keV±0.1 keV. The range of energies may have an upper limit of 100.4 keV−0.5 keV+5 keV. The range of energies may have an upper limit of 100.4 keV±0.5 keV. The range of energies may have an upper limit of 100.4 keV±0.25 keV. The range of energies may have an upper limit of 100.4 keV±0.1 keV.
The emissions of the first type may be uranium K X-ray emissions. The emissions of the first type may be emissions caused by movement of electrons between shells and/or orbits. The emissions of the first type may be uranium Kα emissions, particularly uranium Kα1 emissions. The emissions of the first type may be K X-ray emissions, particularly Kα1 emissions, from a uranium atom caused by alpha and/or beta emissions from another uranium atom. The emissions of the first type may be K X-ray emissions, particularly Kα1 emissions, from a uranium atom caused only by alpha and/or beta emissions from another uranium atom. The emissions of the first type may be emissions arising from self-stimulation of the uranium and ideally only by this mechanism.
Preferably the emissions of the first type are emitted by uranium.
The first type value may be expressed as a count rate or count or intensity or peak height or peak area.
The first type value may be corrected.
The first type value may be corrected for attenuation by the matrix. The first type value may be corrected for detector efficiency and/or attenuation, particularly relative to different detector efficiencies and/or attenuation at other energies.
The first type value may be corrected to remove emissions, for instance of other types and/or from other sources. The first type value may be corrected to remove emissions from other elements than uranium. The first type value may be corrected to remove emissions other than uranium K X-ray emissions and in particular other than Kα1 emissions. The first type value may be corrected to remove emissions from plutonium and/or stimulated by emissions from plutonium. The first value may be corrected to remove plutonium K X-ray emissions and/or to remove K X-rays arising from the decay of plutonium and/or to remove uranium K X-rays caused by stimulation other than by uranium, for instance by plutonium. The first type value may be corrected to remove emissions from and/or stimulated by emissions from caesium and in particular from Cs137. The first value may be corrected to remove uranium K X-rays caused by stimulation by caesium.
The first type value may be corrected to remove emissions other than those arising from the uranium and caused only by alpha and/or beta emissions from the uranium. The first type value may be corrected to remove emissions other than those arising from the uranium caused by alpha and/or beta emissions from another uranium atom. The first type value may be corrected to remove emissions other than those arising from self-stimulation of the uranium. The first type value may be corrected to remove K X-ray emissions, potentially Kα emissions, ideally Kα1 emissions, arising from stimulation of the uranium by alpha and/or beta and/or gamma emissions from other than uranium. The first type value may be corrected to remove K X-ray emissions, potentially Kα emissions, ideally Kα1 emissions, arising from stimulation of the uranium by alpha and/or beta and/or gamma emissions from plutonium and/or caesium.
The correction may be provided by establishing the amount in the matrix in respect of one or more other sources. The correction may involve the removal from the first type value of a correction proportional to the emissions within the first energy range which would arise from the established amount of that other source. The amount of the other source in the matrix may be established by measuring the emissions at at least one energy characteristic of that other source, and ideally at at least one energy characteristic of that other source only. The value for the other source emissions may be expressed as a count rate or count or intensity or peak height or peak area. The value of the correction may be obtained by the known ratio of the other source emissions at the characteristic energy to the emissions from that other source which would therefore arise or would be generated by that other sources action on other material, particularly uranium, at an energy or energies falling within the first energy range. Such a correction may be provided in respect of one or more other sources, and in particular in respect of plutonium and/or caesium.
The method may include measuring one or more energies characteristic of other sources. Where the measurement indicates the presence of one or more other sources, the method may include measuring emissions of a fourth energy, the emissions including emissions of a first type, to provide a further first type value. The measurement of emissions of a fourth energy may be performed instead of or in addition to the measurement of emissions of the first energy. The fourth energy is preferably an energy to which the one or more other sources do not contribute and/or do not cause the uranium to contribute. The further first type value obtained by the measurement of the fourth energy may be used instead of correcting the first type value. Preferably the interrelationship of the values for the further first type value, second type value and third type value then provide information on the dispersion of the radioactive material within the matrix. The further first type value, second type value and third type value may be used without having and/or referring to the first type value.
The fourth energy may be 110.43 keV and/or 111.31 keV and/or 114.34 keV and/or 114.57 keV. The emissions of the fourth energy may be measured by measuring a range of energies. Preferably the range of energies includes 110.43 keV and/or 111.31 keV and/or 114.34 keV and/or 114.57 keV. Where one of these fourth energy values is included in a range, the range may have limits of ±1 keV or ±0.5 keV or ±0.25 keV relative to this. Where the range of energies includes two or more of these fourth energy values, the lower limit of the range may be −1 keV or −0.5 keV or −0.25 keV relative to the lowest of the fourth energy values included in the range and/or the upper limit may be +1 keV or +0.5 keV or +0.25 keV relative to the highest of the fourth energy values included in the range
Preferably the second energy is 89.953 keV and/or 89.9 keV and/or 84.2 keV. The emissions of the second energy may be measured by measuring a range of energies. Preferably the range of energies includes 89.953 keV and/or 89.9 keV and/or 84.2 keV. The range of energies may have a lower limit of 88.6 keV±0.5 keV. The range of energies may have a lower limit of 88.6 keV±0.25 keV. The range of energies may have a lower limit of 88.6 keV±0.1 keV. The range of energies may have an upper limit of 91.1 keV±0.5 keV. The range of energies may have an upper limit of 91.1±0.25 keV. The range of energies may have an upper limit of 91.1 keV±0.1 keV. The range of energies may have a lower limit of 83 keV±0.5 keV. The range of energies may have a lower limit of 83.2 keV±0.25 keV. The range of energies may have a lower limit of 84 keV±0.1 keV. The range of energies may have an upper limit of 85.5 keV±0.5 keV. The range of energies may have an upper limit of 85.2 keV±0.25 keV. The range of energies may have an upper limit of 84.4 keV±0.1 keV.
Preferably the emissions of the second type are thorium K X-ray emissions and/or emissions from thorium, more particularly thorium K X-ray emissions and/or emissions from Th231. The emissions of the second type may be emissions caused by movement of electrons between shells and/or orbits and/or emissions, particularly caused by nucleus decay. The emissions of the second type may be thorium Kα emissions, particularly thorium Kα2 emissions.
Preferably the emissions of the second type are emitted by a decay product of uranium, more particularly of U235.
The second type value of the second energy may be expressed as a count rate or count or intensity or peak height or peak area.
The second type value may be corrected.
The expression may be corrected for attenuation by the matrix. The second type value may be corrected for detector efficiency and/or attenuation, particularly relative to different detector efficiencies and/or attenuation at other energies.
The second type value may be corrected to remove emissions from other sources and/or of other types. The second type value may be corrected to remove emissions from other elements than thorium. The second type value may be corrected to remove emissions other than thorium K X-ray emissions and/or Th231 emissions, and in particular other than Kα2 thorium emissions and/or Th231 emissions. Particularly where the second type value includes an energy of 84.2 keV, the second type value may be corrected to remove emissions from lead. The second type value may be corrected to remove lead K X-ray emissions, particularly lead Kβ emissions and most particularly lead Kβ emissions of energy 84.6 keV.
The correction may be provided by establishing the amount in the matrix in respect of one or more other sources. The correction may involve the removal from the second type value of a correction proportional to the emissions within the second energy range which would arise from the established amount of that other source. The amount of the other source in the matrix may be established by measuring the emissions at at least one energy characteristic of that other source, and ideally at at least one energy characteristic of that other source only. The energy may be 87.4 keV, potentially ±0.1 keV. The value for the other source emissions may be expressed as a count rate or count or intensity or peak height or peak area. The value of the correction may be obtained by the known ratio of the other source emissions at the characteristic energy to the emissions from that other source which would therefore arise or would be generated by that other sources action on other material, particularly uranium, at an energy or energies falling within the first energy range. Such a correction may be provided in respect of one or more other sources, and in particular in respect of plutonium and/or caesium.
The second type value may be isolated from any contribution other than from thorium K X-ray emissions and/or emissions from thorium by a collimator. The detector for the second type of emissions may be provided within the collimator. The collimator may be a lead collimator. The collimator may be lined with copper or tin and preferably both. The copper and tin may be provided as layers, ideally approximately 1.5 mm thick. Preferably the copper is provided closest to the detector.
Preferably the third energy is 92.368 keV and/or 92.8 keV. The emissions of the third energy may be measured by measuring a range of energies. Preferably the range of energies includes 92.368 keV and/or 92.8 keV. The range of energies may further include one or more of 92.29 keV and or 93.35 keV and/or 95.868 keV and/or 94.66 keV. The range of energies may have a lower limit of 91.2 keV±0.5 keV. The range of energies may have a lower limit of 91.2 keV±0.25 keV. The range of energies may have a lower limit of 91.2 keV±0.1 keV. The range of energies may have an upper limit of 96.7 keV±0.5 keV. The range of energies may have an upper limit of 96.7±0.25 keV. The range of energies may have an upper limit of 96.7 keV±0.1 keV.
Preferably the emissions of the third type are emissions from thorium, more particularly from Th234.
Preferably the emissions of the third type are emitted by a decay product of uranium, more particularly of U238.
The method may include obtaining a measure of the third type, and preferably using that to provide the third type value. The method may include processing the measure of the third type to give a third type value.
The measure of the third type and/or the third type value may be expressed as a count rate or count or intensity or peak height or peak area.
The measure of the third type and/or the third type value may be corrected. The expression may be corrected for attenuation by the matrix. The third type value may be corrected for detector efficiency and/or attenuation, particularly relative to different detector efficiencies and/or attenuation at other energies.
The processing of the measure of the third type to give a third type value may include separation of the emissions of the third type from other emissions. In particular the processing may separate emissions arising from and/or indicative of U238 from other emissions. In particular the processing may separate emissions arising from and/or indicative of Th234 from other emissions. In particular the processing may remove emissions arising from and/or indicative of U235 and/or uranium K X-ray emissions and/or thorium K X-ray emissions and/or protactinium K X-ray emissions.
The processing may include determining the emissions attributable to U238 according to the formula:—
                                                                          U            ⁢                                                  ⁢            238                          ⁢                  C                      ROI            ⁢                                                  ⁢            2                          =                ⁢                              C                                      ROI              ⁢                                                          ⁢              2                                      TOTAL                                  -                                    (                              U                ⁢                                                                  ⁢                235                                      ⁢                                          C                                  ROI                  ⁢                                                                          ⁢                  1                                            ×                                                B                                                        ROI                    ⁢                                                                                  ⁢                    2                                                                              U                      ⁢                                                                                          ⁢                      235                                                                                          B                                                        ROI                    ⁢                                                                                  ⁢                    1                                                                              U                      ⁢                                                                                          ⁢                      235                                                                                              )                    -                                                ⁢                              (                          UK              ⁢                                                          ⁢              α              ⁢                                                          ⁢              1                                ⁢                                    C                              ROI                ⁢                                                                  ⁢                3                                      ×                                          B                                                  ROI                  ⁢                                                                          ⁢                  2                                                                      UK                    ⁢                                                                                  ⁢                    α                    ⁢                                                                                  ⁢                    2                                                                                B                                                  ROI                  ⁢                                                                          ⁢                  3                                                                      UK                    ⁢                                                                                  ⁢                    α                    ⁢                                                                                  ⁢                    1                                                                                  )                    where                TOTALCROI2=Net peak area counts for the third type value energy range;        U235CROI1=Net peak area counts for the second type value energy range;        UKα1CROI3=Net peak area counts for the first type value energy range;        U235BROI2=Total U235 branching intensity in the third type value energy range;        U235BROI1=Total U235 branching intensity in the second type energy range;        UKα2BROI2=UKα2 branching intensity in the third type energy range;        UKα1BROI3=UKα1 branching intensity in the second type energy range.        
The interrelationship of the values for the first type value, second type value and third type value which provides information on the dispersion of the radioactive material within the matrix may be defined asUKα1CROI3=a×U235CROI1+b×U238CROI2 
The values of constants a and b may be determined through experimentation and/or calculation and/or modelling. In particular the value of the constants may be determined through a method according to the second aspect of the invention. The values of the constants a and b may be different for different enrichments and/or fissile contents of radioactive material.
The dispersion format of the radioactive material may be expressed in terms of the relationship:
      F    RATIO    =                                                        UK          ⁢                                          ⁢          α          ⁢                                          ⁢          1                    ⁢              C                  ROI          ⁢                                          ⁢          3                            (                        a          ⁢                      ×                          U              ⁢                                                          ⁢              235                                ⁢                      C                          ROI              ⁢                                                          ⁢              1                                      +                  b          ⁢                      ×                          U              ⁢                                                          ⁢              238                                ⁢                      C                          ROI              ⁢                                                          ⁢              2                                          )      
An expression of the dispersion format of radioactive material within a matrix for one matrix may be compared with an expression of the dispersion format of radioactive material within a matrix for another matrix. A higher value for the expression may indicate a less dispersed form. A lower value for the expression may indicate a more dispersed form. The expression may be for an unknown radioactive material in matrix dispersion format and may be compared with a known radioactive material in matrix dispersion format. A relative indication of dispersion may thus be provided.
According to a second aspect of the invention we provide a method of establishing an interrelationship between the values for a first type value, a second type value and a third type value with isotopic composition of a radioactive material the interrelationship providing information on the dispersion of the radioactive material within the matrix, the method including:
defining a form for the interrelationship;
providing a plurality of different isotopic compositions of the radioactive material in a matrix and for each of the different isotopic compositions:                measuring emissions of a first energy, the emissions including emissions of a first type, to provide a first type value;        measuring emissions of a second energy, the emissions including emissions of a second type, to provide a second type value;        measuring emissions of a third energy, the emissions including emissions of a third type, to provide a measure of the third type, processing the measure of the third type to give a third type value;        
adjusting the value of one or more factors in the interrelationship to fit the interrelationship to the observed values for the first type value, second type value and third type value with varying isotopic composition.
The method may further include using the values for the factors to determine the value of the interrelationship in respect of one or more unknown radioactive materials in a material where the radioactive material is of unknown isotopic composition. The relationship may be a linear equation. The factors may be a and b in a formula defined as:UKα1CROI3=a×U235CROI1+b×U238CROI2 
The method may include the use of the interrelationship to provide information on the dispersion of the radioactive material within the matrix according to the form:
      F    RATIO    =                                                        UK          ⁢                                          ⁢          α          ⁢                                          ⁢          1                    ⁢              C                  ROI          ⁢                                          ⁢          3                            (                        a          ⁢                      ×                          U              ⁢                                                          ⁢              235                                ⁢                      C                          ROI              ⁢                                                          ⁢              1                                      +                  b          ⁢                      ×                          U              ⁢                                                          ⁢              238                                ⁢                      C                          ROI              ⁢                                                          ⁢              2                                          )      
Different values for the factors may be established for different dispersion formats of the radioactive material.
The second aspect of the invention may include any of the features, options or possibilities set out elsewhere in this document.
According to a third aspect of the invention we provide a method for providing information on the dispersion of a radioactive material within a matrix, the method comprising:—
measuring emissions of a first energy, the emissions including emissions of a first type, to provide a first type value;
measuring emissions of a second energy, the emissions including emissions of a second type, to provide a second type value;
the interrelationship of the values for the first type value and second type value providing information on the dispersion of the radioactive material within the matrix.
The interrelationship may be the ratio of the first type value to the second type value of vice versa.
In a first particularly preferred embodiment the invention may provide that the first energy is a range of energies. The range of energies may have a lower limit of 96.9 keV±0.5 keV. The range of energies may have a lower limit of 96.9 keV±0.25 keV. The range of energies may have a lower limit of 96.9 keV±0.1 keV. The range of energies may have a lower limit of 94 keV±0.5 keV. The range of energies may have a lower limit of 94 keV±0.25 keV. The range of energies may have a lower limit of 94 keV±0.1 keV. The range of energies may have an upper limit of 100.4 keV−0.5 keV+5 keV. The range of energies may have an upper limit of 100.4 keV±0.5 keV. The range of energies may have an upper limit of 100.4 keV±0.25 keV. the range of energies may have an upper limit of 100.4 keV±0.1 keV. Preferably the second energy is a range of energies. The range of energies may have a lower limit of 88.6 keV±0.5 keV. The range of energies may have a lower limit of 88.6 keV±0.25 keV. The range of energies may have a lower limit of 88.6 keV±0.1 keV. The range of energies may have an upper limit of 91.1 keV±0.5 keV. The range of energies may have an upper limit of 91.1±0.25 keV. The range of energies may have an upper limit of 91.1 keV±0.1 keV. The invention may provide that the amount of radioactive material is determined, preferably based upon the first type value and/or second type value. The amount of radioactive material may be corrected, preferably to account for self-shielding of emissions from it. The correction may be based upon an interrelationship of the values for the first type value and second type value and preferably a ratio between them.
In a second particularly preferred embodiment the invention may provide that the first type value is corrected to remove emissions, for instance of other types and/or from other sources. The first type value may be corrected to remove emissions from other elements than uranium. The first type value may be corrected to remove emissions other than uranium K X-ray emissions and in particular other than Kα1 emissions. The first type value may be corrected to remove emissions from plutonium and/or stimulated by emissions from plutonium. The first value may be corrected to remove plutonium K X-ray emissions and/or to remove K X-rays arising from the decay of plutonium and/or to remove uranium K X-rays caused by stimulation other than by uranium, for instance by plutonium. The first type value may be corrected to remove emissions from and/or stimulated by emissions from caesium and in particular from Cs137. The first value may be corrected to remove uranium K X-rays caused by stimulation by caesium. The first type value may be corrected to remove emissions other than those arising from the uranium and caused only by alpha and/or beta emissions from the uranium. The first type value may be corrected to remove emissions other than those arising from the uranium caused by alpha and/or beta emissions from another uranium atom. The first type value may be corrected to remove emissions other than those arising from self-stimulation of the uranium. The first type value may be corrected to remove K X-ray emissions, potentially Kα emissions, ideally Kα1 emissions, arising from stimulation of the uranium by alpha and/or beta and/or gamma emissions from other than uranium. The first type value may be corrected to remove K X-ray emissions, potentially Kα emissions, ideally Kα1 emissions, arising from stimulation of the uranium by alpha and/or beta and/or gamma emissions from plutonium and/or caesium. The correction may be provided by establishing the amount in the matrix in respect of one or more other sources. The correction may involve the removal from the first type value of a correction proportional to the emissions within the first energy range which would arise from the established amount of that other source. The amount of the other source in the matrix may be established by measuring the emissions at least one energy characteristic of that other source, and ideally at at least one energy characteristic of that other source only. The value for the other source emissions may be expressed as a count rate or count or intensity or peak height or peak area. The value of the correction may be obtained by the known ratio of the other source emissions at the characteristic energy to the emissions from that other source which would therefore arise or would be generated by that other sources action on other material, particularly uranium, at an energy or energies falling within the first energy range. Such a correction may be provided in respect of one or more other sources, and in particular in respect of plutonium and/or caesium. The first type value may be corrected for detector efficiency and/or attenuation, particularly relative to different detector efficiencies and/or attenuation at other energies.
In a third particularly preferred embodiment the invention may provide that the second type value is corrected to remove emissions from other sources and/or of other types. The second type value may be corrected to remove emissions from other elements than thorium. The second type value may be corrected to remove emissions other than thorium K X-ray emissions and/or Th231 emissions, and in particular other than Kα2 thorium emissions and/or Th231 emissions. Particularly where the second type value includes energy of 84.2 keV, the second type value may be corrected to remove emissions from lead. The second type value may be corrected to remove lead K X-ray emissions, particularly lead Kβ emissions and most particularly lead Kβ emissions of energy 84.6 keV. The correction may be provided by establishing the amount in the matrix in respect of one or more other sources. The correction may involve the removal from the second type value of a correction proportional to the emissions within the second energy range which would arise from the established amount of that other source. The amount of the other source in the matrix may be established by measuring the emissions at at least one energy characteristic of that other source, and ideally at at least one energy characteristic of that other source only. The energy may be 87.4 keV, potentially ±0.1 keV. The value for the other source emissions may be expressed as a count rate or count or intensity or peak height or peak area. The value of the correction may be obtained by the known ratio of the other source emissions at the characteristic energy to the emissions from that other source which would therefore arise or would be generated by that other sources action on other material, particularly uranium, at an energy or energies falling within the first energy range. Such a correction may be provided in respect of one or more other sources, and in particular in respect of plutonium and/or caesium. The second type value may be isolated from any contribution other than from thorium K X-ray emissions and/or emissions from thorium by a collimator. The detector for the second type of emissions may be provided within the collimator. The collimator may be a lead collimator. The collimator may be lined with copper or tin and preferably both. The copper and tin may be provided as layers, ideally approximately 1.5 mm thick. Preferably the copper is provided closest to the detector. The second type value may be corrected for detector efficiency and/or attenuation, particularly relative to different detector efficiencies and/or attenuation at other energies.
In a fourth particularly preferred embodiment, the invention may provide that the second energy includes 84.2 keV. The emissions of the second energy may be measured by measuring a range of energies. The range of energies may include 84.2 keV. The range of energies may have a lower limit of 83 keV±0.5 keV. The range of energies may have a lower limit of 83.2 keV±0.25 keV. The range of energies may have a lower limit of 84 keV±0.1 keV. The range of energies may have an upper limit of 85.5 keV±0.5 keV. The range of energies may have an upper limit of 85.2 keV±0.25 keV. The range of energies may have an upper limit of 84.4 keV±0.1 keV. The range of energies may have an upper limit of 91.1 keV±0.5 keV. The range of energies may have an upper limit of 91.1±0.25 keV. The range of energies may have an upper limit of 91.1 keV±0.1 keV. The range of energies may have an upper limit of less than 94.6 keV, for instance an upper limit of 93.85 keV±0.5 keV. The range of energies may have an upper limit of 93.6±0.25 keV. The range of energies may have an upper limit of 93.5 keV±0.1 keV. The second type value may be corrected to remove emissions from lead. The second type value may be corrected to remove lead K X-ray emissions, particularly lead Kβ emissions and most particularly lead Kβ emissions of energy 84.6 keV.
In a fifth particularly preferred embodiment, the invention may provide that the second energy includes 143 keV. The emissions of the second energy may be measured by measuring a range of energies. The range of energies may include 143 keV. The range of energies may have a lower limit of 132 keV±2 keV. The range of energies may have a lower limit of 135 keV±2 keV. The range of energies may have a lower limit of 138 keV±1 keV. The range of energies may have an upper limit of 150 keV±5 keV. The range of energies may have an upper limit of 150 keV±2 keV. The range of energies may have an upper limit of 148 keV±1 keV.
The third aspect of the invention may include any of the features, options or possibilities set out elsewhere in this document.
According to a fourth aspect of the present invention we provide a method for providing information on the dispersion of a radioactive material containing uranium within a matrix, the method comprising:                measuring emissions of a first energy indicative of dispersion of the uranium, to provide a first type value;                    measuring emissions of a second energy indicative of U235, to provide a second type value;            measuring emissions of a third energy indicative of U238 to provide a third type value;            the interrelationship of the values for the first type value, second type value and third type value providing information on the dispersion of the radioactive material within the matrix.                        
The fourth aspect of the invention may include any of the features, options or possibilities set out elsewhere in this application, including in the first and/or second and/or third aspects of the invention.